Brightness > 1 was implemented as a white fullscreen overlay at alpha
(br-1), so the whole image lerped toward white — desaturating and washing
everything out (fully white at max). Add a multiplicative brightness
pipeline to OverlaySystem using a dst-color blend
(result = dst*src + dst, src = br-1) so the scene is multiplied by br —
true brightening that saturates highlights but keeps color, like a real
brightness/gamma control. Darkening already used a black overlay, which is
a correct scene*br multiply, so it's unchanged.
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